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Metal Casting Technologies : September 2007
88 BacktoBASICS C The importance of pouring temperature control ontrolling the temperature at which molten metal is poured into moulds is critical to the production of quality, defect free castings. All castings should be poured within a specified temperature range which is controlled by regular measurement. The consequence of variations to established pouring temperatures is invariably defects in castings which can range from minor surface blemishes to gross defects causing rejection. Some of the major defects caused by excessively cold or hot pouring temperatures are: DEFECTS CAUSED BY LOW POURING TEMPERATURES ■ Low metal temperatures cause a reduction in fluidity resulting in defects ranging from lack of definition to incomplete filling. Fig. 1. ■ In the case of grey and ductile iron, low pouring temperatures can result in cope surface blow-holes and dross defects. ■ Lustrous carbon film inclusions. ■ Inadequate fusion of chaplets and inserts. Fig. 2 ■ Low metal temperatures are often the cause of feeders failing to pipe resulting in shrinkage defects. ■ White iron structures and under-cooled graphite in thin sections of iron castings. Fig. 3. ■ Hot tears. Pouring at low temperatures increases risk of hot tears due to inadequate burn out and collapse of mould and core binders. DEFECTS CAUSED BY HIGH POURING TEMPERATURES ■ Surface roughness. Fluidity increases with temperature so high metal temperatures can more easily penetrate the voids of surface sand grains resulting in a rough surface. ■ Slag inclusions. High metal temperatures increase the fluidity of slags making them more prone to be carried through the gating system into the casting. ■ Fusion defects such as burn on and burn in. ■ Expansion defects such as veins are exacerbated by high metal temperatures due to increased sand expansion. Fig. 4. ■ Metal penetration. High pouring temperatures increase the fluidity of metals so they are more prone to penetration. ■ Shrinkage defects in "hot-spots" such as re-entrant angles and in front of gates. By J. F. Meredith, Casting Solutions Pty Ltd, Moorebank, NSW, Australia www.metals.rala.com.au Figure 1. Incomplete filling of brass casting Figure 2. Inadequate fusion of chaplet Figure 3. Chill in corner caused by low metal temperature